This invention relates to novel porous bodies which possess a low density and a high surface area as well as one or more other beneficial properties such as pore volume and strength characteristics, which makes them suitable for many industrial applications, such as insulating materials, fibers, absorbents, adsorbents, ion-exchange resins, membranes and support materials for a wide variety of uses. The porous bodies have an open-celled 3-dimensional lattice structure.
Numerous attempts have been made to make low density solid materials. For instance, U.S. Pat. No. 4,110,164 to Sutthoff et al. teaches a porous granule produced by mixing a modified cellulose and a densification agent into an agglomerated fibrous ion exchange composite that may be useful in iramobilizing enzymes.
U.S. Pat. No. 4,675,113 to Graves et al. discloses porous beads comprising calcium alginate and magnetite that are suitable for affinity chromatography. The porous beads are formed by dripping an admixture of alginic acid and magnetite into a calcium chloride solution. The alginic/magnetite admixture may further contain triethanolamine titanium, a crosslinking agent, to provide additional physical strength.
Aerogels are examples of highly porous materials. Aerogels have been made from iron and tin oxides, aluminas, tungsten, biopolymers and, more commonly, silicas. The first aerogels were produced in the early 1930's (Kistler, Stanford University) by exchanging the water in an aqueous solution of sodium silicate with an alcohol, and then removing the alcohol under high temperature and pressure (81 bars, 240.degree. C.). The aerogels have densities in the range of 0.03 to 0.3 g/cm.sup.3. Recently, Hrubesh of The Lawrence Livemore National Laboratory modified the technique by using a condensed silica form, base catalyst and supercritical fluid extraction to achieve porous solids of silica aerogels having ultra low density of about 0.005 g/cm.sup.3 (See, Robert Pool Science, 247 (1990), at 807). One disadvantage of such materials is that at these densities the porous solids have limited strength properties. Secondly, the aerogels can be somewhat difficult to modify (chemically) for various commercial applications. Another disadvantage of the Hrubesh method is the use of the expensive supercritical fluid extraction procedure.
Others have attempted to crosslink polymeric gel materials, such as chitosan. For example, Japanese Patent Publication No. 61-133143, published Jun. 20, 1986, and U.S. Pat. No. 4,833,237 to Kawamura et al. disclose crosslinked granular bodies derived from a low molecular weight chitosan. The process for producing the chitosan bodies comprises dissolving a low molecular weight chitosan into an aqueous acidic solution, pouring the solution into a basic solution to form porous, granular gel bodies of chitosan, thoroughly replacing the water contained in the granular gel bodies with a polar solvent, and then crosslinking the granular bodies with an organic diisocyanate. However, it has been found that the water-solvent replacement process causes a significant portion of the pores, especially fine pores, to collapse, preventing the crosslinking agent from having access to form crosslinks. Consequently, the resultant product is swellable and has significantly reduced surface area.